Net Assets Investment Company, Portfolio Turnover C000025078 [Member] American High-Income Municipal Bond Fund® - Class A C000025080 [Member] American High-Income Municipal Bond Fund® - Class C C000025081 [Member] American High-Income Municipal Bond Fund® - Class F-1 C000068565 [Member] American High-Income Municipal Bond Fund® - Class F-2 C000179998 [Member] American High-Income Municipal Bond Fund® - Class F-3 No Trading Symbol Flag C000114834 [Member] American High-Income Municipal Bond Fund® - Class R-6 Bloomberg Municipal Bond Index [Member] Investment Type [Axis] U.S. Treasury and agency [Member] AAA/Aaa [Member] AA/Aa [Member] A/A [Member] BBB/Baa [Member] Below investment grade [Member] Unrated [Member] Short Term Securities And Other Assets Less Liabilities [Member] Short-term securities & other assets less liabilities [Member]